The raider achievement is obtained by destroying the last enemy protector in 10 dungeons. Once obtained, a player will be 20% of the way towards the Master Raider achievement.
It is impossible to complete this achievement without having (at the very least) level 3 engineering skill. Most dungeons, however, will require level 5 engineering skill.
Master Raider Achievement
Destroy the last protector in 50 dungeons.
Enemy protectors are small reddish devices found only in dungeons. They protect the dungeon but can be destroyed if a player has a high enough engineering level. Level 5 engineering is required to destroy all types of enemy protectors. Generally, dungeons have more than one protector. Only destroying the final protector will count as raiding the dungeon and increase progress towards the raider achievements.
You don't have to kill the guardian brains to raid a dungeon -- it's the protectors that matter.
Note that bunkers and dungeons in hell biomes may be covered by an infernal protector (sometimes in addition to regular enemy protectors). Infernal protectors look and function differently than regular enemy protectors. For more information, see the Expiator page.
Originally, it only required 30 raids to complete the master version of this achievement.